Auxiliary power units are mounted aboard many types of aircraft to supply pneumatic and/or shaft horsepower to drive accessories such as electric generators or hydraulic pumps. At the core of the auxiliary power unit is a small gas turbine comprised in flow series arrangement of a compressor, a combustor, and a turbine. The turbine is coupled to the compressor and to a gearbox via a rotating shaft. Pneumatic power in the form of bleed air is extracted from the gas turbine engine through a bleed port positioned between the compressor and combustor. When pneumatic power is required the bleed port is opened and pressurized air is bled off from the engine. However, when only shaft horsepower is required the bleed port is closed. The closing of the bleed port backpressures the compressor and may drive it into a surge condition. To prevent the surging of the compressor a surge bleed valve is operably disposed within the the bleed port and opens to relieve the backpressure by bleeding off air and dumping it overboard. This dumped air is lost energy which must be compensated for by increasing the fuel flow to the engine.
Often, the type of compressor used in these engines is a centrifugal or radial type compressor. Centrifugal compressors include an impeller mounted for rotation in a support housing. The housing also defines a diffuser extending radially outward from the impeller's exit and having a plurality of fixed diffuser vanes disposed therein. As the impeller rotates, a stream of air or other gas is generated that flows from the inlet of the impeller to the impeller's exit. The kinetic energy of the rotating impeller is transferred to the gas stream resulting in high velocity gas exiting the impeller. The diffuser reduces the high absolute velocity of the gas and converts its kinetic energy into static head or pressure.
While these compressors operate over a variety of flow conditions and ranges, they are designed to operate most efficiently at one set of operating conditions, usually referred to as the design point. In the case of Auxiliary Power Units, the compressor is designed for maximum efficiency and minimum adequate surge margin when operating with the bleed port closed and supplying maximum shaft horsepower. As a consequence of selecting these design conditions, when the bleed port is opened the compressor is operating off design and at reduced efficiency. It has long been recognized, that a compressor's efficiency off design can be improved by varying the diffuser area as the operating point of the compressor changes while still maintaining adequate surge margin.
Some have proposed devices for varying the width of the diffuser as the compressor's operating point changes. Examples of such systems can be found in U.S. Pat. Nos. 4,932,835, 4,884,690, 4,616,483, 4,527,949, and U.S. Pat. No. 4,503,684. However, because of the mechanical complexity of these devices they are not suited for use aboard an aircraft where light weight and high reliability are critical requirements.
Others have proposed devices that rotate or pivot the diffuser vanes to vary the throat area of the diffuser as the operating point changes and thereby maintain sufficient surge margin. Examples of these systems can be found in U.S. Pat. Nos. 4,737,071, 4,718,819, 4,554,325, 4,338,063, 4,325,673, and U.S. Pat. No. 3,992,128.
In general, one shortcoming of these prior art devices is aerodynamic inefficiency caused by leakage of gas around the pivoted diffuser vanes from their pressure side to their suction side. This leakage results from the necessity of maintaining an operating clearance between the vanes and the side walls. This clearance must be large enough to permit the vanes to freely rotate, and also be large enough to accommodate any thermal growth or distortion in the vanes and walls that may occur during the operation of the engine. Further, leakage occurring in the vicinity of the leading edge of the vane is most troublesome as that is where the largest pressure differential occurs. One approach to reducing this leakage is taught by Hall, U.S. Pat. No. 4,325,673 which discloses diffuser vanes having elastomeric inner portions which expand to seal against the diffuser side walls as the vanes are rotated.
Another deficiency in the prior art devices is their inability to accurately pivot all of the vanes to the same angle due to backlash and mechanical play in the linkages connecting the vanes to an actuator. The backlash and mechanical play is generally caused by the need for large operating clearances to overcome thermal growth and misalignment of the linkages caused from excessive friction between linkages and inherent errors in manufacturing processes used to manufacture the various components.
Accordingly, there is a need for an improved variable diffuser vane assembly in which the shortcomings of the prior art devices are minimized.